Anyone who has built or lives in a Passive House building already has this part of the energy transition taken care of. After all, the low energy demand in a Passive House can sustainably come from regional energy sources.
The supply structure is transitioning from fossil sources to renewables at an encouragingly rapid pace. The old assessment systems for energy demand in buildings are based on the old supply system and do not work in the new one. The Passive House Institute therefore developed a new evaluation system based on renewable primary energy (PER, Primary Energy Renewable). It also takes proper account of the energy that a building generates. This new evaluation system consists of three Passive House classes:
The Passive House Classic, which is the traditional Passive House
The Passive House Plus, in which additional energy is generated, such as from photovoltaics. Such buildings are said to produce about as much energy as residents consume..
In a Passive House Premium, far more energy is produced than needed. It is therefore a goal for the particularly ambitious: building owners and designers who want to go beyond what economic and ecological considerations already propose.
The new classes – most things unchanged ()!
Most people probably think of a single number when they hear the term Passive House: 15 kWh/(m²a). It describes the maximum demand for annual heating energy for compliance with the Passive House Standard; this is a very low consumption compared with ordinary buildings – the reduction factor is at least four and may be up to ten.
This figure is still included in all of the classes in the new evaluation system because it provides a starting point by limiting the amount of useful energy made available for heating purposes indoors. Useful energy demand for cooling, airtightness, and criteria for comfort and hygiene also remain the same. The experience with these criteria are good – including best possible comfort, reliable low consumption and affordable measures to achieve these.
But heating energy demand does not tell the whole story; after all, heating energy demand is roughly equal to hot water demand in a Passive House. Demand for household electricity is usually much higher in such energy efficient buildings. A building’s total energy demand – including the energy needed to provide the building with final energy – therefore also needs to be taken into account. This is where the new Passive House classes come in. They divide buildings into categories based on renewable primary energy demand and their own renewable primary power production (Figure 1).
Generation and demand remain separated
Power from a solar roofis considered primary electricity with a PER of 1.0. It is exported to the grid and not calculated directly against the building’s energy demand. The PER model is used to calculate demand. For example, solar power generated in the summer should not be treated as though it directly offsets heating energy in the winter because energy from the summer would need to be stored seasonally for the winter, a process that entails additional losses. If this factor is not taken into consideration during planning, buildings are not properly optimized. By taking account of renewable primary energy, the new system allows the building to be made future proof.
Energy generation relative to the building’s ground area
Often, energy demand and generation are stated with reference to a building’s treated floor area. If a building has a photovoltaic array, it can produce a certain amount of energy, but the amount per square meter of floor area decreases as the number of stories (and hence floor area) increases. Single-story bungalows thus seem to perform better than row houses and duplexes/complexes, although bungalows actually consume much more area and resources per resident.
Stating renewable energy production in terms of floor area can thus also lead to improper optimizations. In the new concept, energy generation is instead stated relative to the building’s ground area, defined as the vertical projection of the thermal envelope towards ground. Whether a bungalow or a complex is built, the assessment is therefore the same in terms of energy generation. This approach is better because the space a building takes up is then no longer available for other types of usage. If this area is used to generate electricity, there are additional benefits, and these benefits are then assessed in terms of this area. After all, the sun shines on the roof, not on the treated floor area on every story.
Using biomass budgets efficiently
Both within Germany and worldwide, biomass is only available in limited amounts. There is a clear usage hierarchy for biomass: 1) food production, 2) materials, and 3) energy. Because biomass can be stored and has a high energy density, it will mainly be needed in mobile applications (transport). Only a small amount will be left over for consumption in buildings. And because biomass can be stored, it is perfect for use in the winter. The budget is then prioritized as follows: heating, hot water in the winter, and household electricity.
Note that it is more efficient to generate electricity with biomass first and then use a heat pump for heat supply second. If some of the biomass is combusted in a household stove, around 80 percent of the primary energy can be converted into useful heat. If biomass is consumed in a cogeneration unit, around 50 percent of the energy is used to produce electricity and 30 percent to produce useful heat, with only 20 percent losses.
A heat pump allows three units of heat to be generated from a single unit of electricity. In this case, 50 percent electricity becomes 150 percent heat in addition to the 30 percent useful heat from the cogeneration unit. As a result, biomass produces 180 percent useful heat in combination with a heat pump instead of 80 percent useful heat from direct combustion. Nonetheless, Passive House buildings can continue to have biomass heating systems; the overall PER demand will simply be relatively high in such cases.
The sun and wind provide primary electricity. Some of this electricity can be used directly. However, storage capacities are necessary for transferring surplus energy to time periods with lower energy gains. These supply secondary electricity as required, and this is associated with losses. Depending on the type of energy application, the proportion of primary and secondary electricity varies, as do the losses for providing energy. These specific energy losses of an energy application are described as the respective PER factor. The demand for domestic energy is quite constant throughout the year, which is why the share of direct electricity is high and the PER factor is low. In contrast with this, heating is necessary only in winter. In order to provide enough energy in winter, electricity must in part be produced in summer and stored with very high losses for the winter, which results in a high PER factor.
With the passive house concept it is possible to realize an energy system, which is fully relying on renewable energy generation – and the energy storage systems necessary in the grid are available with well known technology and affordable. The passive house thus opens the path to a sustainable energy supply.
Prof. Dr. Wolfgang Feist,
Originator of the Passive House concept,
Founder and director of the Passive House Institute in Darmstadt/Germany and Innsbruck/Austria,
Professor at the University of Innsbruck – Unit Energy Efficient Buildings